Subscriber-controlled registration technique in a CDMA system

ABSTRACT

A subscriber controlled registration protocol, a subscriber monitors a congestion indicator signal broadcasted by a base station with which it desires to register. If the congestion indicator signal indicates that the base station is operating in a congested state, the mobile station selects another base station in the system. Otherwise, it attempts to register with the first selected base station.

BACKGROUND

The present invention provides a subscriber-implemented registrationtechnique for use in a CDMA communication system to uniformly distributesubscriber load among base stations.

Typically in a CDMA system, when a subscriber terminal (“mobilestation”) begins operation, it registers with a base station as part ofits initialization operation. As part of registration, the mobilestation typically identifies a pilot channel of a base station andcommunicates with the base station to identify the mobile station'spresence in the base station's cell. The mobile station typicallytransmits a registration request message on an access channel associatedwith the base station or its pilot channel. When multiple pilot channelsare detected, the mobile station typically registers with the basestation whose pilot channel is associated with a highest qualityreception.

As is known, CDMA communication systems are “interference limited.”Unless determined by a communication protocol, a base station is notcharacterized by a fixed number of communication channels by which itmay communicate user data to mobile stations. In theory, a “congested”base station may continue to add communication channels to satisfyincreased demand for service, but the communication quality of eachchannel in the system would be diminished incrementally. In practice,the number of channels that may be satisfied by a base station isdetermined by minimum call quality standards that are to be maintainedby the communication system. As is known, this number of channels alsomay be affected by environmental conditions in the cell that maycontribute to quality degradations. However, it is desirable to limitunnecessary communication in a congested CDMA cell to improve the callquality of communication channels already in process and to reduceundesirable cross-channel interference.

In traditional CDMA systems, a mobile station will attempt to registerwith a base station based solely on the mobile station's measurement ofthe received signal strength, E_(c)/I_(o) or SNR of the pilot channels.Although provisions exist in some CDMA systems for base stations torefuse to register a mobile station based upon congestion levels of thebase station, registration in such systems typically includes a firsttransmitted registration request message from the mobile station to thecongested based station followed by a second transmitted message fromthe congested base station to the mobile station denying registration.The mobile station then would attempt to register with the base stationassociated with the next strongest pilot channel. And, in systemsemploying soft handoff, if the mobile station succeeds in registeringwith the next strongest base station, the mobile station may retryregistration with the congested base station upon expiration of acountdown timer.

The registration request and denial transmissions that are used in thesetraditional CDMA systems consume precious bandwidth in analready-congested base station. They contribute to cross-channelinterference with other channels already in progress and further consumeprocessing resources in the already-congested base station. Accordingly,there is a need in the art for a low bandwidth registration protocol ina CDMA system, one that reduces processing demands upon a base stationoperating in a congested state.

SUMMARY

Embodiments of the present invention provide a subscriber controlledregistration protocol in which a subscriber monitors a congestionindicator signal broadcasted by a base station with which it desires toregister. If the congestion indicator signal indicates that the basestation is operating in a congested state, the mobile station selectsanother base station in the system. Otherwise, it attempts to registerwith the first selected base station.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a CDMA communication system.

FIG. 2 is a flow diagram of a mobile station operating in accordancewith a first embodiment of the present invention.

FIG. 3 is a flow diagram of a mobile station operating in accordancewith a second embodiment of the present invention.

DETAILED DESCRIPTION

The present invention relates to a registration technique for use inCDMA communication systems in which a mobile station determines which ofa plurality of base stations it will register with. The mobile stationmakes its decision based on loading indicators transmitted globally froma base station to subscriber stations.

FIG. 1 illustrates a typical CDMA communication system 100. Thecommunication system may be populated by a plurality of base stations100-160. The base stations 100-160 communicate with a plurality ofmobile stations such as the mobile station 170 shown in FIG. 1. Eachbase station typically broadcasts a plurality of logical channels on aphysical channel including a pilot channel, a “sync” channel, a pagingchannel and a plurality of traffic channels.

As is known, the pilot channel may be thought of as a “beacon”transmitted from a base station. Typically, the pilot channelconstitutes a data signal having a predetermined pattern. The pilotchannel typically carries no information. Mobile stations use the pilotchannel on initialization to acquire carrier phase and timingrelationships. Often, the same CDMA code that identifies the pilotchannel of a first base station also is used to identify the pilotchannel of other base stations. However, in such systems, neighboringbase stations transmit the CDMA code using predetermined delay offsetswith respect to each other that provide clear discrimination betweenbase stations. However, the predetermined delay relationships ofadjacent pilot channels permits a mobile station to quickly acquire thepilot channels of adjacent base stations once it has acquired the pilotchannels of a first base station.

A sync channel may be used by the base station to communicateadministrative information to a mobile station. For example, a basestation may transmit a base station ID to a user, a color code andadministrative information identifying system status. Sync channels aretransmitted globally within the cell; typically, mobile stations thatare idle (are not engaged in active communication connections) monitorthe sync channel.

Each base station may transmit one or more paging channels. Pagingchannels typically are used to command a mobile station to set uptraffic channels. Traffic channels may carry user data to mobilestations when the mobile stations participate in active communicationconnections.

According to the principles of the present invention, the communicationprotocol of the sync channel may be modified to include a congestionindicator signal that identifies whether the base station is operatingin a congested state. In a first simplest embodiment, the congestionindicator field simply may include a flag signal. When the base stationis not operating in a congested state, the flag signal may be set to afirst predetermined state indicating that a mobile station may attemptto register with the base station. When the base station is operating ina congested state, the flag signal may be set to a second predeterminedstate indicating that the mobile station should not attempt to registerwith the base station.

According to this first embodiment of the present invention, mobilestations may register in accordance with the method 1000 illustrated inFIG. 2. Upon start up, a mobile station 170 typically monitors receivedtransmissions and searches for pilot channels of neighboring basestations (Step 1010). The number and identity of detected pilot channelsmay depend upon the topology of the system, the mobile station'slocation therein, and environmental conditions. Typically, when itdetects a pilot channel from a first base station, a mobile station mayexploit the predetermined timing offsets among the pilot channels in thesystem to facilitate acquisition of the pilot channels from other basestations. Depending upon ambient conditions, a mobile station may notdetect pilot channels from all the base stations in a system. These areknown characteristics of conventional CDMA systems.

The mobile station may rank the base stations based upon the quality ofthe detected pilot channels (Step 1020). As a first indicator ofquality, the mobile station may rank the base stations based upon areceived power level associated with each pilot channel, called the“received signal strength indicator” (or “RSSI”) in connection with someknown CDMA systems. Alternatively, the mobile station may rank the basestations based upon a measured bit error rate (“BER”) for the pilotchannels. Further, the ranking of base stations may be made on the basisof ratios of energy per chip to aggregate received energy (commonlyrepresented as E_(c)/I_(o)). And, of course, the ranking may beperformed based upon some combination of RSSI and BER and E_(c)/I_(o)measurements.

The mobile station selects the base station that was ranked highest inpilot quality (Step 1030). It monitors the sync channel to acquire thecongestion indicator signal (Step 1040) and determines whether the flagsignal therein indicates that the base station is accepting additionalregistrations from mobile stations (Step 1050). If the base station isaccepting new registrations, the mobile station attempts to registerwith the selected base station (Step 1060). If not, the mobile stationselects the base station that appears next in its ranking of pilotquality (Step 1070). Thereafter, it returns to step 1040, monitors thatcongestion indicator signal from the newly selected base station anddetermines whether to register to it.

Theoretically, if severe loading conditions were present throughout agiven system 100, it is possible that a mobile station would cycleinfinitely through a loop created by traversing steps 1040-1050 and1070, then returning again to step 1040. According to an embodiment ofthe present invention, a mobile station may be programmed to discontinuethe method of FIG. 2 if it cycles through steps 1040-1050 and 1070 apredetermined number of times without identifying a base station thatwill accept new registrations. Alternatively, the mobile station may beconfigured to discontinue the method of FIG. 2 if, at step 1070, themobile station selects a base station that is associated with pilotquality that is below a predetermined call quality threshold establishedfor the system. As another alternative, the mobile station may interruptthe loop of steps 1040-1050 and 1070 if it traverse the entire set ofpilot channels acquired at Step 1010.

Registration as represented in Step 1060 may be accomplished accordingto any of a number of well-known registration schemes and may includeadditional functionality not discussed herein.

In an alternate embodiment of the system 100, base stations may beconfigured to broadcast congestion indicator signals that report notonly whether the transmitting base station is in a congested state butalso identify a neighboring base station that is operating in a lightlycongested state. In such an embodiment, registration of a mobile stationmay operate in accordance with the method 2000 of FIG. 3.

According to the method 2000, a mobile station acquires pilot channels,ranks the pilot channels and monitors congestion indicator signals as isdescribed above with respect to Steps 1010-1050 in FIG. 2 (Steps2010-2050). If the highest quality base station is acceptingregistrations of new mobile stations, the mobile station registers withthe base station (Step 2060).

If the base station is operating in a congested state and is notaccepting new registrations from mobile stations, the mobile stationidentifies a lightly loaded base station from the congestion indicatorsignal transmitted by the congested base station (Step 2070). The mobilestation may determine if the measured pilot quality of the lightlyloaded base station exceeds minimum call quality thresholds for thesystem (Step 2080). If so, the mobile station registers with the lightlyloaded base station (Step 2090). Otherwise, the mobile station selectsthe base station of the next highest pilot quality and returns to Step2040 (Step 2100).

In an embodiment of the present invention, at Step 2080, the mobilestation may simply retrieve the pilot quality measurement that had beenobtained according to the acquisition and ranking steps of Steps 2010and 2020. Having previously measured the quality of all pilot channelsthat were detectable, it is not necessary according to this embodimentfor the mobile station to reacquire the pilot channel of the lightlyloaded base station prior to attempting registration in Step 2090.

Optionally, however, according to other embodiments of the presentinvention, the mobile station may first monitor the congestion indicatorof the lightly loaded base station prior to attempting registration. Inthis alternative, instead of advancing from Step 2080 to Step 2090, themobile station may simply select the lightly loaded base station (Step2110 in phantom) and return to step 2040. In this alternative, inaddition to receiving the congestion indicator from the lightly loadedbase station, the mobile station also may perform quality measurementsupon the sync channel (step not shown). This alternate embodiment enjoysthe additional advantage of permitting the mobile station to adapt tochanging conditions in the communication system. If the lightly loadedbase station experiences a sudden congestion event or if signal qualityfrom the lightly loaded base station suddenly becomes unacceptable, themobile station may determine not to attempt registration to that basestation.

As described above, a congested base station may include an identifierof a lightly loaded base station in the system. The identifier may berepresented as an integer representing the differential delay offsetbetween the pilot of the congested base station and the lightly loadedbase station. As described above, different base stations in a CDMAsystem typically transmit the same pilot signal but at large relativedelay offsets. For example, adjacent base stations participating in theknown IS-95 cellular system transmit pilot channels that are shiftedwith respect to each other by an integer multiple of 64 pilot codechips. IS-95 specifies a “PNINCREMENT” setting that determines the shiftincremented of the pilot codes. According to an embodiment of thepresent invention, the identifier of the lightly congested base stationmaybe transmitted as an increment identifier representing an integernumber of these predetermined shifts. In such an embodiment, the mobilestation may attempt to acquire a new pilot channel shifted with respectto the present base station's pilot channel by the increment identifier.

According to another embodiment of the present invention, the identifieroptionally also may include a color code. If the mobile station receivesa color code in the congestion indicator signal, it may receive the syncchannel associated with any pilot received at the identified offset andcompare a color code received therein with the identified color code. Ifthe two color codes do not match, the mobile station may abortregistration attempt and advance to step 2100 instead (step not shown).

The registration protocol of the present invention provides an importantadvantage of reducing signal interference in a CDMA cell. In the presentinvention, the mobile station makes a determination of which basestation it will attempt to register with based upon administrativeinformation transmitted by base stations. The mobile station does notbegin transmission until it has determined which base station it willregister with.

This technique reduces interference in a congested cell by eliminatingthe registration request and rejection messages that would otherwise betransmitted in the cell.

Thus, the present invention contributes to reduced interference in aCDMA system by providing registration decision making in mobile stationsrather than base stations.

The subscriber controlled registration techniques of the presentinvention may find application in any of a number of CDMA communicationsystems. It may be integrated into the known IS-95 cellularcommunication protocol with minor modifications to the communicationprotocol of the IS-95 sync channel protocol. Further, it may be used inother CDMA communication systems such as the Tanlink communicationsystem currently under development by the assignee of the presentinvention, Tantivy Communications, Inc. The Tanlink system ischaracterized as a “nomadic access” system. Mobile stations in presentiterations of the Tanlink system typically possess directional antennaeand register to one and only one base station. The subscriber controlledregistration techniques of the present invention also may findapplication in other CDMA systems not described herein.

Several embodiments of the present invention are specificallyillustrated and described herein. However, it will be appreciated thatmodifications and variations of the present invention are covered by theabove teachings and within the purview of the appended claims withoutdeparting from the spirit and intended scope of the invention.

We claim:
 1. A registration method in a mobile station, comprising:acquiring pilot channels in a communication system, ranking the pilotchannels according to quality of reception for each pilot channel,selecting a highest of the ranked pilot channels, receiving a congestionindicator signal associated with the selected pilot channel,transmitting a registration request signal on an access channelassociated with the selected pilot channel when the congestion indicatorsignal is in a first state, and repeating until all acquired pilotchannels are exhausted.
 2. A registration method in a mobile station,comprising: acquiring pilot channels in a communication system, rankingthe pilot channels according to quality of reception for each pilotchannel, selecting a highest of the ranked pilot channels, receiving acongestion indicator signal associated with the selected pilot channel,transmitting a registration request signal on an access channelassociated with the selected pilot channel when the congestion indicatorsignal is in a first state, and repeating until a pilot channel isselected that is associated with a quality of reception that is below apredetermined threshold.
 3. A registration method in a mobile station,comprising: acquiring pilot channels in a communication system, rankingthe pilot channels according to quality of reception for each pilotchannel, selecting a highest of the ranked pilot channels, receiving acongestion indicator signal associated with the selected pilot channel,transmitting a registration request signal on an access channelassociated with the selected pilot channel when the congestion indicatorsignal is in a first state wherein the congestion indicator signal isreceived in a sync channel associated with a pilot channel.
 4. Theregistration method of claim 3, wherein the quality of reception of apilot channel is determined by a bit error rate of the pilot channelmeasured at the mobile station.
 5. The registration method of claim 3,wherein the quality of reception of a pilot channel is determined byE_(c)/I_(o).
 6. A registration method in a mobile station, comprising:acquiring pilot channels in a communication system, ranking the pilotchannels according to quality of reception for each pilot channel,selecting a highest of the ranked pilot channels, receiving a congestionindicator signal associated with the selected pilot channel, andtransmitting a registration request signal on an access channelassociated with the selected pilot channel when the congestion indicatorsignal is in a first state wherein the congestion indicator signal isreceived in a paging channel associated with a pilot channel.
 7. Aregistration method in a mobile station, comprising: acquiring pilotchannels in a communication system, ranking the pilot channels accordingto quality of reception for each pilot channel, selecting a highest ofthe ranked pilot channels, receiving a congestion indicator signalassociated with the selected pilot channel, transmitting a registrationrequest signal on an access channel associated with the selected pilotchannel when the congestion indicator signal is in a first state, andfurther comprising, when the congestion indicator signal is not in afirst state, receiving an identifier of a second pilot channel, andtransmitting a registration request message on a access channelassociated with the second pilot channel.
 8. The registration method ofclaim 7, further comprising: receiving a congestion indicator signalassociated with the second pilot channel, and if the quality ofreception associated with the second pilot channel exceeds apredetermined threshold, transmitting a registration request message onan access channel associated with the second pilot channel.